Numerous devices have been employed for locking longitudinally slidable members in preselected positions. A commonly used system is a set screw which is threadably received in one member and adapted to be advanced tightly against the second member to prevent relative sliding movement. Some such set screws employ a knob which can be manually turned so that a screwdriver or other such implement is not needed.
Other systems employed for releasably retaining longitudinally slidable members in preselected positions include a button attached to one of the slide members and biased away therefrom so as to fit into one of a plurality of openings provided in the other of the members to prevent relative sliding movement. The button is depressible to remove it from the opening so that the members can be slid relative to each other into different positions.
One other system for releasably locking longitudinally slidable members in preselected positions utilizes a multipiece unit having a button protruding through one of the slidable members at an easily accessible location and a second button protruding through one of a plurality of holes in the other sliding member in a nonvisible position so that upon depression of the first button the second button is removed from its locking position in one of the openings whereby the members can be slid until a desirable relative relationship is obtained. Upon obtaining the new relationship the first button can be released to allow the second button to protrude into another hole in the second member. This system, however, consists of several component parts which make it relatively expensive to manufacture and more susceptible to malfunction than a simpler system.